HSN has been featuring a really nice Proform for awhile, has lots a features and is in your price range. I have been really tempted, but got an elliptical instead. Check out HSN.com, maybe they have it there.
My mom recently bought a treadmill to walk on. I was shopping around with her prior to purchase and from what I was seeing, there is not much offered for a runner for $500 or less. If you are mainly interested in walking you will have a much easier time of it. I'm not saying there is nothing out there...it'll just be a tougher find.
I bought my Weslo treadmill about 9 years ago at WalMart for around $400. It has been a great investment for me and an asset to my running. I've logged numerous running miles on it without any problems. Near the end of this year I plan to replace it for something with more bells and whistles. (My treadmill has an incline button, it does not tell me how much the incline is.) At that time I will be able to afford to spend more money than I could in the past.
I've heard great things about ProForm. It seems like Sears is always running a special on them. You might want to check it out.
I looked at Sears when I bought mine. They always have sales but I believe Dick's has better prices. Do you have a Dick's near you? Mine is also a Proform. I can't remember how much we spent on it but it is wonderful for running. HTH
LD